1. Field of the Invention
The present invention relates to a proton-conducting polymer suitably usable as an electrolyte for hydrogen fuel cells, direct methanol fuel cells, and the like.
2. Description of the Related Art
A fuel cell is produced by the steps of interposing an electrolyte between an anode and a cathode, sandwiching thus obtained electrolyte-electrode joined assembly between a pair of separators to form a unit cell, and stacking a predetermined number of such unit cells. The electrolyte may be a proton-conducting polymer, which functions to transfer protons generated on the anode to the cathode.
A perfluoropolymer and an organic substance prepared by adding sulfonic acid to an aromatic hydrocarbon are known as the proton-conducting polymer. The sulfonic acid-added organic substance cannot show a sufficient proton conductivity singly, and thereby is used under wet condition to achieve a practically preferred conductivity. Therefore, in the case of using the proton-conducting polymer of the sulfonic acid-added organic substance as the electrolyte in the fuel cell, a humidifier is used for humidifying both reaction gases (a fuel gas for the anode and an oxidant gas for the cathode) to maintain the electrolyte under the wet condition.
When moisture is excessively supplied to the reaction gases, reaction gas passages are disadvantageously obstructed by the moisture, whereby the supply rates of the gases are lowered to cause deterioration in the electricity generation of the fuel cell. A method for appropriately controlling such a moisture supply amount is proposed in Japanese Laid-Open Patent Publication No. 2002-280029.
When the fuel cell is used at an environmental temperature below the freezing point, the moisture supplied to the reaction gases is frozen. In this case, to prevent the freeze, the fuel cell is generally heated by a large external heater and then driven (see Japanese Laid-Open Patent Publication No. 2006-260962).
As described above, when the fuel cell containing the electrolyte of the sulfonic acid-added organic substance is driven, it is necessary to strictly control the reaction gas humidity and the driving temperature. Further, since the external humidifier and heater are needed, the fuel cell system is disadvantageously large, resulting in increased equipment investment.